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Topic: [SOLVED]What recourse when electronic components are out of tolerance? (Read 2534 times) previous topic - next topic

boggydew

Oct 15, 2016, 09:23 am Last Edit: Oct 18, 2016, 03:51 am by boggydew Reason: SOLVED sort of.
Hi,

I recently had 100 MCPCB's made that had Vishay 15V zener P.no. BZG03C15 on board.  The zener is passing 25mA and does not even get warm to touch, yet the voltage across them is 15.6V or more.  According to the data sheet, 15.6V is the tolerance maximum with a test current of 50mA, I'm only passing half that.  As such my boards, which cost me $3500 are scrap.  What recourse is available to consumers when components are supplied out of tolerance?

Cheers.

weedpharma

This is something to take up with the board supplier. It is pointless asking us until you have discussed it with the supplier.

Weedpharma

TomGeorge

Hi,
What is your application that relies on the precision of 15V zener.
What tolerance do you need?
Zeners are not precision devices, any power supply that uses them, has adjustable circuitry to compensate for tolerance and temperature drift.
Did your prototype use the Vishay devices?
You could inquire from your supplier about them, are they the real deal component?

Tom.... :)
Everything runs on smoke, let the smoke out, it stops running....

raschemmel

Without seeing your schematic it is impossible to be sure, but is it possible your current limiting resistor for the zener is not allowing enough current to the zener to obtain the 50 mA test current ? It appears to be a 3W diode. Why is it only drawing 25 mA ?
Can you post the portion of the schematic that contains the zener circuit ?

Vishay is a good vendor. I would not be inclined to suspect the problem is a quality issue. I would be more inclined to suspect a design issue.

Quote
Did your prototype use the Vishay devices?
If you don't mind me asking, didn't your engineering tech submit a report for breadboarded test circuit results ? How are you just now discovering this AFTER the PCB was made and not BEFORE ? Isn't that the purpose of building bench prototypes in the first place ? I realize their are SOME circuits that CANNOT be breadboarded. A 15V zener is not one of them.
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Wawa

This is a "15volt standoff" TVS diode, primarily meant for overvoltage/spike protection of e.g. a 15volt supply.
AFAIK not ok for voltage regulation.
Can you show us that part of the circuit you have used it for.

What is stopping you from replacing that part on the board.
Or piggy-back a second (different) diode on top.
100 boards shouldn't take more than a few hours to fix.
Leo..


boggydew

What is stopping you from replacing that part on the board.[/quote]

Aluminium PCB, closely populated.  Its not like re-flowing an FR4 board with your hairdryer.
Without seeing your schematic it is impossible to be sure, but is it possible your current limiting resistor for the zener is not allowing enough current to the zener to obtain the 50 mA test current ?
That's the point I was making.  As current increases through a zener, the wattage increases, the temp increases, the zener voltage increases.  If the zener is 15.6V at 25ma, it can only be higher at 50mA, which puts it out of tolerance.

Simple 12V battery solar panel charging regulator.  Panels have an open circuit voltage of 21V, passing 25mA they are at 20V. So 20V - 15V = 5V. 5V/180ohms = 28mA . Panel + to collector of NPN and resistor. Junction of resistor and zener to Base. Emitter to Battery + through a diode.  15V - 0.6V for diode - 0.6V for BASE/Emitter diode drop = 13.8V max at battery +. I tested it with prototype FR4 boards and it worked spot on.  I concede the design mistake, I should have designed at tolerance maximum not at nominal voltage, but the fact that these zeners perform outside tolerance only makes the situation worse..

MarkT

Replacing components on a 100 boards is perfectly doable with someone experienced in hot-air rework work,
scrapping may be avoidable.

If you want precision voltages the components needed are a precision voltage reference and precision
resistor divider network.  Zeners and TVS's in particular are very soft knee.  Never confuse a "typical"
graph on the datasheet for guaranteed values.  The breakdown ratings you describe are pulse ratings,
which means self-heating effects are avoided - continuous currents will cause self heating and voltages
will not conform to the pulse-rating value.  25mA at 15V means  nearly 0.4W are being dissipated in
the TVS die, so there is significant self heating in your application.

The other issue you have to be aware of is counterfeit or sub-spec components being used by
unscrupulous companies - this is a big problem and its also possible you have been a victim.

[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

Grumpy_Mike

Quote
According to the data sheet, 15.6V is the tolerance maximum with a test current of 50mA, I'm only passing half that.
Therefore you can't say the part is out of specification. Passing lees that the test current will mean that the zener voltage will be greater than it would be if you were to be passing the test current.

I think you problem is that you don't understand zener diodes.

raschemmel

As I said, Vishay is a known good manufacturer.  The PhD Electrical Engineers at my company spec Vishay parts all the time.  I am more inclined to suspect the design, which is prooving to be the problem. Looks like someone is going to being some rework. Our rework person can have all 100 boards done by lunchtime.
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allanhurst

If you weant an accurate 15v ref shunt regulator you'd be much better usiing eg a TL431

if you want it spot-on use SOT or a potentiometer.

regards

Allan

Boardburner2

The other issue you have to be aware of is counterfeit or sub-spec components being used by
unscrupulous companies - this is a big problem and its also possible you have been a victim.

Who did you use for the assembly work ?

Ask them if they have documentation showing the provenence of the diodes.
A reputable house should have these records.

MrAl

Hi there,

Despite what you might have heard, most diodes above about 5 or 6 volts show a POSITIVE temperature coefficient, which means that as they get hotter the voltage rises even without increasing the current.  That means that at 25ma if you read 15.6v then at 50ma there's no way you are going to read a LOWER voltage, as it will naturally go higher because diodes of the higher voltage variety work on a different principle than diodes under 5 or 6 volts.  A diode under that voltage will exhibit a NEGATIVE temp co which means the voltage will go down with temperature, but since you have a 15v zener it is most likely going to increase in voltage as the temperature goes up.
There will be a negative feedback effect with that too however, because as the voltage goes up there is less and less voltage headroom for the current limiting resistor which means the current will fall somewhat, depending on the resistor and main power source, so the power will not increase as much, but it may still increase, again depending on the diode itself and the other circuit components.

In short however for a 15v zener, the voltage will increase as the current increases and will increase a little more because of the positive temp co, so there's no way it will go down at some point.  This means that some sort of modification is required.

I dont know the exact circuit so it's hard to say at this point, so perhaps you can post the circuit and also what exactly is expected of it under all expected operating conditions.  That would help figure out the simplest fix.  It would also be good to know if you can solder another part in parallel on the back of the board as that sometimes helps too.

As many people here noted already, a zener is not the right way to provide accurate voltage regulation or protection.  Too many ways the voltage regulation point can change.


raschemmel

Quote
Therefore you can't say the part is out of specification. Passing lees that the test current will mean that the zener voltage will be greater than it would be if you were to be passing the test current. 
I'm not an expert on zeners but my understanding was the zener does not clamp at the zener voltage until it reaches the minimum required current. (pretty much what Mike said)
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boggydew

MrAl,  thank you for your supportive response. Grumpy Mike needs to read it, perhaps it is he who doesn't understand zeners.

I posted to see if anyone had a similar experience of having boards manufactured with out of spec components, but most people would rather pick at my design to bolster their own egos.

If you weant an accurate 15v ref shunt regulator you'd be much better usiing eg a TL431

if you want it spot-on use SOT or a potentiometer.

regards

Allan

Thanks for your suggestion Allen.  I'm looking into it.  What do you mean by SOT (habitual drunkard?)

One other thing I don't understand about my whole experience with zeners and their tolerance rating is this.  All the BZG03C15  I used for prototyping zenered at 15V+/-1%, while all the BZG03C15 on my manufactured boards zener at 15.6V +/-1%.  The data sheet says tolerance is between 14.4V to 15.6V, and as such you would expect to see a random assortment of voltages within this range for each individual component.  But I only see very tight tolerances within a particular batch, and the wider tolerance between batches.  Why don't they bin them into tighter tolerances?  I have seen the same phenomena with different part number zeners as well.

raschemmel

SOT-23 SOT-323 etc

Try using Google once in a while

SOT = "small outline transitor" (not "device")

Some people just can't admit when they've made a mistake. A TVS diode is not a regulation diode.
If it's a 3W diode it should be conducting more than 25 mA. ( like at least the Test Current, 50 mA)
Look at the SMAZ5929B
Look at the maximum current (75 mA) Voltage (15.2 V)
Look at the test current (25 mA)
 A BSEE would know that. Why is it wrong for me to tell you this ?
YOU are the one who posted asking US to tell you what you did wrong ( but phrasing the post title like it couldn't possibly be your mistake)
If you are posting on an expert forum I can't understand how you can be upset about being told you specc'd the wrong part. Is that or is that not our JOB (and DUTY) to tell you that ? WHAT KIND of experts would we be if we didn't tell you because we didn't want to hurt your feelings ? When you go to a doctor or lawyer don't you EXPECT the TRUTH ,the WHOLE TRUTH, and nothing but the TRUTH ?
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